OSV-2020-1687 Bad-cast to pcpp::Layer from invalid vptr

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=25340 Crash type: Bad-cast Crash state: Bad-cast to pcpp::Layer from invalid vptr pcpp::IDnsResource::getRawData pcpp::DnsResource::getDataLength...